In recent years, DPFs have become widely used for collecting and suppressing soot and smoke, particularly from diesel engines, in accordance with regulations regarding automobile emission control.
The DPF is a filter used in an exhaust pipe to collect the soot and smoke. The collected soot on the filter is burned so that the soot will not be released during exhaust. An abnormal burn may cause breakage of the ceramic filter during burning if the amount of the soot on the filter is either too much or too little. In other words, the amount of the collected soot has to be measured precisely in order to control soot burning.
The amount of the collected soot is usually measured by the difference of pressures on both sides of the filter. The pressure loss caused by the filter becomes an indicator of the amount of soot collected on the filter. The pressure loss is captured either as a pre-filtering pressure (upstream pressure) in an absolute pressure detection method, or as a difference of pre/post filtering pressures (a difference between an upstream pressure and a downstream pressure) in a relative pressure detection method.
The pressure sensor used for the relative pressure detection method in a conventional system includes, for example, a diaphragm as a covering of a sensor element as well as a pressure receiver in a sensing portion (refer to Japanese Patent Document JP-A-2003-315193).
FIG. 3 shows a cross-sectional view of the pressure sensor used in the relative pressure detection method.
In this pressure sensor, two concave portions 11a, 11b are disposed on both sides of a case 10 with a connecting through passage. The through passage is blocked by a sensor element 20 in the concave portion 11a. The two concave portions 11a, 11b are filled with oil 70. Two conductive metal diaphragms 81, 82 cover the two concave portions 11a, 11b and seal the oil 70 therein.
In this case, an upstream side of the DPF in the exhaust pipe is, for example, connected to a pressure induction port 12a of a first port portion 12, and a downstream side of the DPF in the exhaust pipe is connected to a pressure induction port 13a of a second port portion 13 respectively through rubber hoses or the like.
Through those hoses, the pressure on the upstream side of the DPF is applied to the first diaphragm 81 in the case 10, and the pressure on the downstream side of the DPF is applied to the second diaphragm 82.
Pressure on both diaphragms 81, 82 is applied to the sensor element 20 through the oil 70, or more precisely, a difference of the pressures on the diaphragm 81 and the diaphragm 82 is received by the sensor element 20. The sensor element 20 outputs an electrical signal proportional to the difference of the applied pressures.
The sensor element 20 in the concave portion 11a and a terminal 10a disposed in the case 10 are electrically connected by a wire 40 to output the electrical signal from the sensor element 20.
However, the pressure sensor shown in FIG. 3 is susceptible to external noise such as an electromagnetic wave and, as a result, the circuit in the sensor may malfunction. The external noise, in this case from an automotive pressure sensor, includes ignition noise from the engine and the like.
The external noise causes an abnormal current in the circuit connected to the sensor element 20. That is, electrical current in the sensor element 20, the wire 40 and/or the terminal 10a suffers from bad influences such as abnormal amplification from the external noise.
Conventionally, the sensor device is shielded from the external noise by wrapping the circuit with a conductive material in order to raise noise-resistibility.
FIG. 3 specifically shows a prototype of a sensor device electrically shielded with a conventional shielding method manufactured by the inventors.
The pressure sensor is enclosed in a conductive member 500 composed of, for example, a metal case, a metal deposition film and the like, with the exception of a portion for the terminal 10a for external connection.
However, the conductive member 500 used for enclosing the pressure sensor for electrically shielding a circuit portion 10a, 40 (combination of the terminal 10a and the wire 40) requires a large number of parts because of the size, and thus increased manufacturing costs.